1. Technical Field of the Invention
The present invention relates to a planar array antenna having microstrip-line antenna elements, which is primarily applied in millimeter wave and microwave bands, and more particularly to a planar array antenna which has an improved high antenna gain and maintains a wide bandwidth.
2. Description of the Related Art
With the developments in radio communication technologies, especially mobile communications, antennas are required to be of higher performance and smaller size. Planar antennas are widely used in millimeter wave and microwave bands. Planar antennas are generally grouped into microstrip-line antennas and slot-line antennas. Of these planar antennas, microstrip-line planar antennas are small in size and can easily be manufactured, and has a feature that it can be produced at low cost and the like. However, since microstrip-line planar antennas have a relatively low antenna gain, it has been customary to construct a microstrip-line planar array antenna using a plurality of the antenna elements. The present inventors have already proposed in Japanese Patent Laid-open Application No. 2003-115717 (JP, P2003-115717A), a planar array antenna which can facilitate impedance mating in a feeding system for a plurality of antenna elements of microstrip-line type and remarkably simplify the constitution of the feeding system.
FIG. 1A is a plan view of a conventional microstrip-line planar array antenna in which the number of antenna elements which are fed is four, and FIG. 1B a cross-sectional view taken along line A—A of FIG. 1A.
On one principal surface of substrate 1 which is made of a dielectric material, antenna elements 2a to 2d each of which is constructed by a square conductor and a feeding system which supplies RF power to the antenna elements 2a to 2d. Each of antenna elements 2a to 2d is an antenna element of a microstrip-line type, and these antenna elements are arranged in a quadruplet manner. The centers of antenna elements 2a to 2d are disposed on the position of apexes of a geometric square, for example, apexes of a certain regular square. Ground conductor 4 is formed on an almost entire surface of the other principal surface of substrate 1. In the example shown here, the antenna elements are arranged in a matrix in two horizontal rows and two vertical columns.
The feeding system comprises microstrip line 3a which is formed, as a first feeding line, on one principal surface of substrate 1, slot line 3b which is formed, as a second feeding line, on the ground conductor in the other principal surface of substrate 1, and microstrip line 3c which is formed, as a third feeding line, on one principal surface of substrate 1.
Microstrip line (i.e., the first feeding line) 3a connects antenna elements disposed adjacent in the right and left direction. Among two microstrip lines 3a, both ends of upper microstrip line 3a are connected to antenna elements 2a, 2b, respectively. Similarly, both ends of lower microstrip line 3a are connected to antenna element 2c, 2d. Slot line (i.e., the second feeding line) 3b both ends which traverse two microstrip lines 3a at the proximity of midpoints of these microstrip lines 3a and are electromagnetically coupled to microstrip lines 3a. Microstrip line (i.e., the third feeding line) 3c extends from the feeding end T disposed at the left end of substrate 1, and the tip end of microstrip line 3c traverses the mid point of slot line 3b and is electromagnetically coupled to slot line 3b. 
In this case, with the wavelength corresponding to antenna frequency (i.e., resonant frequency) taken as λ, the both ends of slot line 3b extend approximately λ/4 in length from the traversing points with upper and lower microstrip lines 3a and become electrically open ends for the resonant frequency component seen from the traversing points. Similarly, the tip end of microstrip line 3c extends approximately λ/4 in length from the traversing point with slot line 3b and becomes electrically an electrically short-circuited end for the resonant frequency component seen from the traversing point.
Explanations will be made for the case of transmission, for example. In such an array antenna, as electric field E illustrated by a arrow mark and a mark indicating the direction against the substrate plane, high frequency power P from feeding end T of microstrip line 3c is first propagated to slot line 3b and then it branches in-phase upper and lower directions on slot line 3b from the midpoint of slot line 3b. That is, the high frequency power branches in-phase from microstrip line 3c to slot line 3b. High frequency power P is then propagated to microstrip line 3a from the end portion of slot line 3b, and it branches in opposite phase in left and right directions on microstrip line 3a from the midpoint of microstrip line 3a. Each of antenna elements 2a to 2d is thus fed through microstrip line 3c, slot line 3b and microstrip line 3a. In the following description, an antenna element which is connected to an end of a feeding line of a microstrip line type to be fed with the high frequency power is referred to as a powered antenna element. Consequently, antenna elements 2a to 2d are powered antenna elements.
As obvious from the figure, since the feeding positions on powered antenna elements 2a, 2c, that is, the connecting positions of microstrip strip lines 3a have a mirror symmetric relation to the feeding positions on powered antenna elements 2b, 2d, each of antenna elements 2a to 2d is excited in-phase. Radio waves having the same polarization plane are emitted from respective antenna elements 2a to 2d in the perpendicular direction and these radio waves are combined. In this case, the electric field plane direction of the radio wave is in the feeding direction of the high frequency power and the magnetic field plane direction is perpendicular to the electric field plane direction. Of course, in the case of reception, this array antenna operates in the same manner as described above.
By comparing one in which the feeding system is arranged with only microstrip lines, this array antenna has a simple configuration for impedance matching and the feeding system of a simple structure. Further, with a configuration in which four antenna elements are arranged in the above manner taken as a basic unit, an array antenna having more number of the powered antenna elements can be configured by combining a plurality of the basic units.
For example, with the four-element array antenna described above taken as the basic unit, an eight-element array antenna can be constructed as shown in FIG. 2A by arranging two pieces of the basic units in mirror symmetry (or point symmetry) around feeding ends T of these basic units as a center, connecting microstrip lines 3c of the two basic units to each other, and electromagnetically coupling slot line 3d as a fourth feeding line to the midpoint of the common-connected microstrip line 3c. 
Further, a 16-element array antenna is constructed as shown in FIG. 2B by preparing two pieces of the eight-element array antennas shown in FIG. 2A, arranging two pieces of the eight-element array antennas in mirror symmetry (or point symmetry) around the feeding ends thereof as a center, connecting slot lines 3d to each other, and electromagnetically coupling microstrip line 3e as a fifth feeding line to the midpoint of the common-connected slot line 3d. The 16-element array antenna shown in FIG. 2B is provided with four pieces of the basic units described above.
An array antenna having further number of antenna elements can be constructed by combining array antennas in the above manner. Specifically, n being integer larger than or equal to 3, an array antenna having 2n+1 pieces of antenna elements is constructed by arranging two pieces of 2n-element array antennas in mirror symmetry or point symmetry around a feeding end of the (n+1)-th feeding line as a center, connecting the (n+1)-th feeding lines of the 2n-array antennas to each other, and providing an (n+2)-th feeding line which traverses a midpoint of the common-connected (n+1)-th feeding line and is electromagnetically coupled to the common-connected (n+1)-th feeding line. In this 2n+1-element array antenna, 2n−1-pieces of the basic units describe above are included. It should be noted that an n-th feeding line is a microstrip line where n is an odd number and the n-th feeding line is a slot line when n is an even number.
The planar array antenna described above has, however, an disadvantage that it basically has a narrow frequency band width because each powered antenna element is an antenna element of a microstrip line type. Therefore, it is proposed in Japanese Patent Laid-open Application No. 2004-328067 (JP, P2004-328067A) to widen the band width of frequency characteristics of the antenna by disposing a passive element ahead of the powered antenna element. FIGS. 3A to 3C illustrate a microstrip line planar array antenna whose frequency band is widen by loading a passive element to each powered antenna element. The passive element means an antenna element of a microstrip line type which consists of a conductor just like the powered antenna element but is not directly connected to a feeding line. The passive element is excited through the electromagnetic coupling with the powered antenna element and emits electromagnetic wave.
The array antenna shown in FIGS. 3A to 3C is one in which passive antenna elements 6a to 6d are loaded to the four-element array antenna shown in FIGS. 1A to 1C. FIG. 3A is a plan view illustrating an intermediate layer in the array antenna, FIG. 3B is a plan view of the array antenna, and FIG. 3C is a cross-sectional view taken along line A—A of FIG. 3A.
Powered antenna elements 2a to 2d are disposed on one principal surface of a first substrate which consists of a dielectric material, and ground conductor 4 is arranged on the other principal surface of the first substrate. Second substrate 1b which consists of a dielectric material is laminated on the one principal surface of first substrate 1a so that second substrate 1b covers antenna elements 2a to 2d. Multilayer substrate 5 is constituted from first substrate 1a and second substrate 1b. Antenna elements 2a to 2d are sandwiched and disposed between first and second substrates 1a, 1 b, and the plane in which antenna elements 2a to 2d are formed is referred to as an intermediate layer. The arrangement of the feeding system for antenna elements 2a to 2d is identical to that shown in FIGS. 1A and 1B.
On the surface of second substrate 1b, passive elements 6a to 6d which are not connected to the feeding system are disposed at the position ahead of powered antenna elements 2a to 2d which are disposed on the intermediate layer so that passive elements 6a to 6d oppose to powered antenna elements 2a to 2d, respectively. It should be noted that a pair of a powered antenna element and a passive element corresponding to the powered antenna element is referred to as a powered element pair. Therefore, the antenna illustrated in the figure is provided with four sets of powered element pairs 26a to 26d. The frequency band of a microstrip line planar array antenna is widen by arranging a passive element ahead of each powered antenna element in this manner. However, in this configuration, as the number of the antenna elements is increased for improving the antenna gain, the number of the basic units described above is also increased. It is required to supply more high frequency power to the antenna.